For obtaining stable and highly efficient energy supply, nowadays, many regenerative energy systems are configured by a bidirectional DC-DC converter with temporary energy storage elements. Due to the energy recovery, the proportion of bidirectional bridge circuits increases continuously. The same have the function of adapting different voltage levels. Accordingly, it can happen in many applications that the areas of input and output voltage of the converter overlap in a charging and discharging energy storage. Consequently, a specific type of DC-DC converters is needed, which can be operated both in the buck mode, i.e. reduction of an input voltage as well as in the boost mode, i.e. increase of an input voltage.
There are cascaded buck-boost converters having four switches that are frequently used for such applications due to simple topology and high load capability. As an interface, in particular between regenerative energy systems and energy storages, the efficiency of the converter has a significant influence on the efficiency of the entire system.
U.S. Pat. No. 6,166,527 presents a control concept for a buck-boost converter for reducing power losses. Depending on the ratio of input and output voltage, the converter is operated in three different modes, in the buck mode, the boost mode or in the buck-boost mode. Thereby, the power losses of the DC-DC converter are reduced in buck and boost modes, but due to the simultaneous operation of the four switches, the losses in the buck-boost mode still remain high.
US 2012/0146594 A1 presents a control concept where the buck-boost converter is operated in five different modes. These are the buck mode, a buck mode with half the switching frequency, the buck-boost mode with half the switching frequency, the boost mode with half the switching frequency and the boost mode. When the input and output voltage approach one another, the switching frequency of the PWM signals (PWM=Pulse Width Modulation) is halved. Thereby, the range of the buck and boost modes can be extended and the range of the lossy buck-boost modes can be reduced accordingly. This enables an increase of the efficiency of the converter in the transition phase between the buck mode and the boost mode but has little energy efficiency in the buck-boost mode.
Therefore, there is a need for concepts of reducing or minimizing the power losses in cascaded buck-boost converters.